A ferroelectric memory element can be modeled as a pair of capacitors with a small differential capacitance, as disclosed in U.S. Pat. No. 5,729,488, the entire contents of which are incorporated herein by reference. The differential capacitance and mean capacitance values change significantly with temperature, voltage, and voltage history. A xe2x80x9c1xe2x80x9d/xe2x80x9c0xe2x80x9d is written into the element or cell containing two ferroelectric capacitors by applying (+/xe2x88x92) write voltage (Vwrite) to one element, and (xe2x88x92/+) Vwrite to the other element. Vwrite is typically the power supply voltage +5V, or +3.3V, etc., so the write driver circuitry can be made of standard CMOS inverters.
To read the data from the memory cell, a sense amplifier applies a read voltage (Vread) to the two ferroelectric elements in the cell, and senses the difference in capacitance between them (Cfxe2x88x92Cfb) by measuring the charge differential resulting from applying the read voltage (Vread). The read voltage (Vread) is typically less than 1V.
If too large a read voltage is applied, the ferroelectric elements will be re-written. If too small a read voltage is applied, the differential capacitance will approach zero. Appropriate values of the read voltage vary with temperature and the manufacturing details of the ferroelectric element. The differential capacitance (Cfxe2x88x92Cfb) is large for the first few reads after a write; for example, the capacitance of one of the elements (Cf) might be twice as large as the capacitance of the second element (Cfb). After many writes, particularly with temperature cycling, Cf might only be a few percent larger than Cfb. To maximize memory density, Cf and Cfb should be as small as possible.
A sense amplifier should be capable of applying a well-controlled read voltage to ferroelectric memory elements, and reliably sensing a wide range of differential capacitance.
A major challenge in designing a non-destructive-read ferroelectric memory is that the differential signal capacitance may vary greatly as a function of temperature, temperature history and the number of times a capacitor has been xe2x80x98readxe2x80x99, among other factors. Because of these ferroelectric capacitor characteristics, a sense amplifier should be capable of correctly sensing the small differential signal capacitance in the presence of large and/or poorly controlled non-signal capacitances. In addition, a sense amplifier should provide adequate sensitivity even where there are component mismatches within the amplifier itself.
Embodiments of the invention are adaptable for use in a radiation-hard ferroelectric memory with good manufacturability, operation over a wide temperature range, and capable of unlimited reads and writes.
Other approaches, xe2x80x9cSawyer-Towerxe2x80x9d (see e.g., C. B. Sawyer, C. H. Tower, xe2x80x9cRochelle Salt as a Dielectric,xe2x80x9d Physical Review, Vol. 35 (February 1930)) and xe2x80x9cShared Chargexe2x80x9d (see e.g., Brennan, xe2x80x9cAnalysis of Electric Fields, Space Charges, and Polarization of Thin Film Ferroelectric Capacitors Based on Landau Theoryxe2x80x9d, MRS Proceedings (Fall 1991), U.S. Pat. No. 5,140,548 (Brennan) and U.S. Pat. No. 5,309,390 (Brennan)), apply a read voltage to ferroelectric memory elements through source follower transistors. Charge is transferred from the ferroelectric elements through the source followers to sense capacitors and the voltage difference across the sense capacitors is amplified to give a digital xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
These approaches have slightly different topologies to produce different signal gains and different sensitivities to interconnect capacitance.
Moreover, the sensitivity of these approaches is limited by transistor mismatches and some are sensitive to unbalanced interconnect capacitance. When manufactured in a modern semiconductor facility, these approaches achieve limited sensitivity and yield.
Exemplary embodiments of this invention are capable of high sensitivity in the presence of normal mismatch of semiconductor devices and interconnect capacitances. They are able to measure a small difference-signal capacitance in the presence of circuit mismatches and large unequal interconnect capacitances. The amplifier compensates for typical manufacturing mismatches that would otherwise produce an erroneous signal. It is suitable for non-destructive sensing of ferroelectric memory capacitors.
An exemplary embodiment of the sense amplifier of this disclosure includes three sections: a common-mode section applies a xe2x80x98readxe2x80x99 voltage to two capacitors in a manner that rejects unequal interconnect capacitance, a difference-mode section generates a signal proportional to the capacitance difference between the two capacitors, and an offset-canceling section compensates for circuit mismatches in the difference-mode section. The two capacitors may be two ferroelectric capacitors comprising a ferroelectric differential capacitance memory unit. The sense amplifier allows improved sensitivity and manufacturability. It allows non-destructive-read ferroelectric memories to achieve desirable yield, density, and operating temperature ranges.